1. Field of Invention
The present invention relates to a lateral diffused metal-oxide-semiconductor (LDMOS) device having an increased punch-through voltage; particularly, it relates to an LDMOS device having an increased punch-through voltage without sacrificing its breakdown voltage. The present invention also relates to a method for making such an LDMOS device.
2. Description of Related Art
An LDMOS device is often used in applications requiring high voltage devices, such as high power and high frequency band power amplifiers. The LDMOS device has a characteristic that it can sustain high voltages from tens to hundreds of volts.
In terms of structure, an LDMOS device is similar to a typical field effect transistor (FET). Both the LDMOS and the FET devices comprise a source and a drain formed in a substrate and separated by a drift region, and a gate structure formed on the drift region. However, the LDMOS device is different from the typical FET device in that, in the typical FET device, the source and drain are symmetric with respect to the gate structure, but in the LDMOS device, the drain is farther from the gate structure as compared to the source.
FIG. 1 shows a prior art LDMMOS device structure, which comprises: a substrate 11, a well 12, an isolation region 13, a body region 14, a source 15, a drain 16, a gate structure 17, and a body contact 19. The substrate 11 is doped with impurities having a conductive type opposite to that of the source 15 and drain 16. Adrift region 21 shown by slash lines is formed between the source 15 and drain 16. A PN junction is formed between the substrate 11 and the well 12, and when the PN junction is reversely biased, a depletion region is formed around the junction as shown by the dash lines in the figure. Once the reverse bias is higher than the punch-through voltage, the depletion region extends to the body region 14, and a leakage current increases abruptly, inducing a punch-through effect from the body region 14 to the substrate 11 and causing damages to the device or malfunction.
As the technology trend requires even smaller device dimension and even higher voltage applied to a device, the aforementioned prior art has to increase the punch-through voltage accordingly. In the aforementioned prior art, a higher punch-through voltage can be achieved by increasing the ion implantation dosage during formation of the well 12. However, this will also decrease the breakdown voltage of the device, limiting the applications of the device.
In view of the foregoing disadvantage in the prior art, the present invention provides an LDMOS device having an increased punch-through voltage without sacrificing the breakdown voltage; the present invention also provides a method for making such device.